The present invention is directed to electrical power extraction from aircraft engines, and more particularly to an electrical power generation system which enables efficient and dynamic production of electrical power from a turbofan engine.
Individual components of turbofan engines require different power parameters during operation. The fan rotational speed, for example, is limited to a degree by the tip velocity and, since the fan diameter is very large, rotational speed must be relatively low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Therefore, separate high and low speed turbines with independent power transmitting devices are necessary for the fan and core compressor in aircraft gas turbine engines. Furthermore since a turbine is most efficient at higher rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the necessary power.
Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand significantly increased electrical power from current commercial airliners as loads transition from hydraulic and pneumatic in nature to electrical. Many mechanical actuators, for example, are being implemented using electric motors, replacing traditional, heavier hydraulic systems. These increased electrical demands require larger generators to supply the additional electrical power.
The increased electrical power demand must be derived from mechanical power extracted from the engines that propel the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may affect the ability to operate the engine to operate properly.
Traditionally, electrical power is extracted from a high-pressure (HP) engine spool in a gas turbine engine. The high and relatively constant operating speed of the HP engine spool, compared to that of the low-pressure (LP) engine spool, makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, this added load placed on the HP engine core, can have a detrimental effect on engine performance under certain conditions.
Drawing power from elsewhere in the engine therefore is necessary in certain cases, and can also be advantageous, by extracting that power in a more fuel-efficient manner. The LP engine spool can provide this alternate source of power. Additionally, by selectively controlling the extraction of electrical power from either the HP or LP engine spools, as desired, benefits in engine performance can be realized.
Specific low-power fault-tolerant permanent magnetic (PM) generator technology installed within a turbofan engine is known. Although fault tolerant PM generators have not yet been used at primary power levels, such generators have been used at low power lever (i.e. less than 500 Watts). There has been however, no use of high power PM generators on turbo fan engines to date, for reasons of cost, complexity, and unproven technology, among other things.
Aircraft engine power extraction control systems are also known. Some of these systems include multiple generators controlled by a central control system. Although 400 Hz systems often run in parallel (aka, military transports and B747-400), variable frequency generators however (non-400 Hz), are not seamlessly paralleled or integrated with one another.
Generator arrangements including LP permanent magnet generators with some form of electrical control system are known in the prior art. None of these arrangements are known to be integrated with other sources (APU or HP driven).
Methods and systems directed to coordinating loads seen by each of the HP and LP generators are known to selectively switch or reduce the loads as the engine conditions permit. These methods and systems are not seamlessly integrated with an aircraft power system.
Multiple (i.e. three) spool engines having a generator on each shaft for supplying power to the engine itself, in addition to an airframe is also known in the art. This technology is not seamlessly integrated with an aircraft power system.
DC generators on LP spools are known in the prior art for providing power to DC busses. These LP generators require a device such as an inverter to interface DC to an AC bus. These approaches disadvantageously require significant integration with an airframe power system, and are not seamlessly integrated with the airframe power system.
In view of the foregoing, it would be advantageous and beneficial to provide a system and method for selectively controlling the extraction of electrical power from either the HP or LP engine spools, as desired, to optimize benefits in engine performance. It would be further advantageous if this system and method were to be seamlessly integrated with an airframe power system.